Volvox is one of the best-known chlorophytes and is the most developed in a series of genera that form spherical colonies. Each mature Volvox colony is composed of numerous flagellate cells similar to Chlamydomonas, up to 50,000 in total, and embedded in the surface of a hollow sphere or coenobium containing an extracellular matrix made of a gelatinous glycoprotein. The cells swim in a coordinated fashion, with distinct anterior and posterior poles. The cells have eyespots, more developed near the anterior, which enable the colony to swim towards light. The individual algae in some species are interconnected by thin strands of cytoplasm, called protoplasmates.
An asexual colony includes both somatic (vegetative) cells, which do not reproduce, and gonidia near the posterior, which produce new colonies through repeated division. The daughter colonies are initially held within the parent coenobium and have their flagella directed inwards. Later, the parent disintegrates and the daughters invert. In sexual reproduction two types of gametes are produced. Volvox species can be monoecious or dioecious. Male colonies release numerous microgametes, or sperm, while in female colonies single cells enlarge to become oogametes, or eggs.
"The most favorable place to look for it is in the deeper ponds, lagoons, and ditches which receive an abundance of rain water. It has been said that where you find Lemna, you are likely to find Volvox; and it is true that such water is favorable, but the shading is unfavorable. Look where you find Sphagnum, Vaucheria, Alisma, Equisetum fluviatile, Utricularia, Typha, and Chara. Dr. Nieuwland reports that Pandorina, Eudorina and Gonium are commonly found in summer as constituents of the green scum on wallows in fields where pigs are kept. The flagellate, Euglena, is often associated with these forms."
Ancestors of Volvox transitioned from single cells to form multicellular colonies at least 200 million years ago, during the Triassic period. Using DNA sequences from about 45 different species of Volvox and related species, a University of Arizona researcher estimates the transition took about 35 million years.
- ^ a b Kirk, David L. Volvox: A Search for the Molecular and Genetic Origins of Multicellularity and Cellular Differentiation. Cambridge University Press, 1998, 399 pp.
- ^ a b c d Single-celled Algae Took The Leap To Multicellularity 200 Million Years Ago from a February 22, 2009 Science Daily article
- ^ Hallmann A. (2003). Extracellular matrix and sex-inducing pheromone in Volvox. Int Rev Cytol. 227: 131-182.
- ^ Ikushima, N, Maruyama S. (1968) The Protoplasmic Connection in Volvox J Eukaryotic Microbiology 15(1): 136-140.
- ^ a b Powers JH. (1908) Further Studies in Volvox, with Descriptions of Three New Species. Trans. Amer. Microscop. Soc. 28: 141-175.
- ^ Chamberlain, Charles Joseph. Methods in Plant Histology University of Chicago Press, 1932, 416 pp.
- ^ van Leeuwenhoek, A. (1700). Phil. Trans. Roy. Soc. 22: 509